Third-seeded Dominika Cibulkova battled past an aggressive Stefanie Voegele on Thursday to set up a quarter-final clash with Urszula Radwanska at the WTA Bank of the West Classic.
Slovakia's Cibulkova let 5-2 leads slip in each set, but emerged with a 7-5, 7-6 (7/5) victory over the Swiss player.
"I expected a really tough match, because she's playing really well right now," Cibulkova said of Voegele, who fired 10 aces in the match.
"I played her a long time ago. I remember she plays close to the lines. I knew it would be a tough match.
"It could have been an easier match for me though -- I was 5-2 up first set and 5-2 up second set -- but in the end I always made it, and even though it could have been easier, I'm still happy with the win," she said.
Seventh-seeded Radwanska of Poland edged Slovakian Daniela Hantuchova 7-6 (7/3), 7-6 (7/3). Hantuchova led 4-1 in the first set and was twice up a break in the second but failed to hold on.
Radwanska trailed 0-3 in the second-set tiebreaker, but won the last seven points to seal the victory.
Fourth-seeded American Jamie Hampton reached the last eight with a hard-fought 7-5, 6-7 (5/7), 6-3 victory over US wild card Nicole Gibbs.
Hampton next faces Russian qualifier Vera Dushevina, who beat American Madison Keys 7-6 (7/0), 6-2, on Friday.
The two remaining quarters will see top-seeded Agnieszka Radwanska, Urszula's sister, face sixth-seeded American Varvara Lepchenko, and Olga Govortsova of Belarus take on fifth-seeded Sorana Cirstea of Romania.